1. Field of the Invention
The present invention relates to an exhaust system for an internal combustion engine of an outboard motor, in which an exhaust passage for leading exhaust gas from the internal combustion engine is formed in a case of the motor.
2. Description of the Related Art
Japanese Patent Publication No. JP-A-Hei 6-66125 describes one type of conventional outboard motor. The outboard motor according to this publication includes a drive unit supported by a hull, a propeller supported by the lower end of the case to be immersed in the water, and an internal combustion engine supported on the upper surface of the drive unit. The drive unit includes an exhaust guide constituting its upper portion, an upper case attached to the lower surface of the exhaust guide, and a lower case attached to the lower surface of the upper case. The lower case is disposed so as to be immersed in water. The propeller is supported by the lower case.
The drive unit is formed with an exhaust passage for leading exhaust gas from the internal combustion engine from an upper side to a lower side of the case and discharging the exhaust gas from the lower side into the water. The drive unit is also formed with a branch passage branching from the exhaust passage at an intermediate portion thereof for discharging the exhaust gas inside the exhaust passage directly out into ambient air. Respective intermediate portions of the exhaust passage and the branch passage are formed with an expansion chamber.
When the internal combustion engine is driven, the propeller is driven in conjunction therewith to propel the boat. At this time, most of the exhaust gas from the internal combustion engine is discharged into the water through the exhaust passage. A portion of the exhaust gas passes through the branch passage from the intermediate portion of the exhaust passage to be directly discharged into the ambient air. When the internal combustion engine is idling, the exhaust gas is small in amount and low in pressure. Therefore, when the exhaust gas reaches the intermediate portion of the exhaust pipe, most of it passes through the branch passage to be directly discharged into the ambient air. On this occasion, the exhaust gas passes through the expansion chambers and the exhaust noise can be thereby reduced.
However, in the conventional outboard motor described above, the expansion chambers are both formed in the upper case. Preferably, the capacities of the expansion chambers are as large as possible in order to sufficiently reduce the exhaust noise. On the other hand, the case, including the upper case, is preferably small so as to facilitate handling of the outboard motor.
Therefore, it is difficult to provide sufficiently large capacities for the expansion chambers. Under such conditions, especially when the internal combustion engine is idling, when most of the exhaust gas passes through the branch passage to be directly discharged into the ambient air, the exhaust noise may not be reduced below a desired level.
Furthermore, when the internal combustion engine is started, only a small amount of exhaust gas passes through the exhaust passage, at a low speed. Thus, a negative pressure may occur in the exhaust passage due to exhaust pulsation at the start, which is likely to cause the water to be drawn into at least the drive unit through the exhaust passage.
The internal combustion engine may sometimes rotate in reverse immediately after it is stopped. In such situations, a negative pressure may occur in the exhaust passage due to the reverse rotation. This negative pressure is likely to cause the water to be drawn into at least the drive unit through the exhaust passage.